Creating low polygon models
By Manipulating primitives.

For instance Tree 1 (Top Left of your ref image)


Add the Tree Trunk.
Do this by adding a cube ans scaling it on Z.
Go into edit mode on the cube, and Select the top face, and scale it inward.
Create a material that looks like the trunk you prefer.
Go back into Object mode.
Create a new Cube.
With this new cube selected go into edit mode, again select the top face and scale it inward until it looks like a pyramid.
Scale up/down according to your preference.
copy the new pyramid upward in Z with Ctrl+D, then Z.
scale the new mesh a little less.
repeat 5-9 one more time.
Create a leaf material and assign it to the three pyramids.


For the next tree copy the trunk of the last tree, then add an Icosphere.

Make sure you have the Tool Panel Open (the panel is on the left of the 3D View, and you can toggle it using the T key.

Then, copy the trunk again, and for the next tree use a lower res icosphere. When you create a new one, look at the lower half of the Tool Panel and set Subdivision to 1.  Then go into Edit Mode, and move, and extrude some thing around scaling them down slightly as you go up the Z axis

For the last tree copy the trunk again, and do some research / experimentation with the Skin Modifier, and when you are done learning how to use it, copy the pyramids from your first tree, and position them accordingly.

I hope this sends you down the right direction, but I must say this kind of question is too broad for this site, and doesn't deserve this kind of attention from the users, and will probably be flagged. Please get a feel for what this site is about by browsing around before asking such a broad question. If you browse you will see more specific questions, and more concise answers.  If you want help on tutorials, and learning try: Blender Artist . org